Ultrasound penetrates well through soft tissues and, due to its short wavelengths, can be focused to spots with dimensions of a few millimeters. As a consequence of these properties, ultrasound can and has been used for a variety of diagnostic and therapeutic medical purposes, including ultrasound imaging and non-invasive surgery of many parts of the body. For example, by heating diseased (e.g., cancerous) tissue using ultrasound, it is often possible to ablate the diseased portions without causing significant damage to surrounding healthy tissue.
The noninvasive nature of ultrasound surgery is particularly appealing for the treatment of brain tumors. Moreover, coherent, non-invasive focusing of ultrasound through the human skull has been considered as a tool for targeted drug delivery to the brain, improved thrombolytic stroke treatment, blood flow imaging, the detection of internal bleeding, and tomographic brain imaging. However, the human skull has been a barrier to the clinical realization of many of these applications. Impediments to transcranial procedures include strong attenuation and the distortions caused by irregularities in the skull's shape, density, and sound speed, which contribute toward destroying the ultrasound focus and/or decreasing the ability to spatially register received diagnostic information.
Several minimally invasive or noninvasive aberration-correction techniques for transskull focusing overcome the focusing difficulties at least partially. Minimally invasive approaches may use receiving probes designed for catheter insertion into the brain to measure the amplitude and phase distortion caused by the skull, and then correct the ultrasound beam using an array of transducers. An alternative, completely noninvasive approach uses X-ray computed tomography (CT) images, rather than receiving probes, to predict the wave distortion caused by the skull.
Noninvasive focusing with a therapeutic array has been demonstrated at frequencies of about 2 MHz with a longitudinal wave propagation model. The velocity of these waves is approximately 2700 m/s in the skull, and about 1500 m/s in water and soft tissue. Due to this ratio, sound that arrives at the skull under an incident angle above about 30°, the critical angle, is reflected. The amplitude of the focus therefore drops when the focus is directed close to the skull surface. As shown in FIGS. 1A and 1B, a treatment envelope 100 is defined as the region accessible to ultrasound from a sufficient number of transducers 102 to enable treatment. Whereas the effect of reflection is minimal when the target area is deep within the brain (FIG. 1A), since it lies inside the treatment envelope 100, reflection becomes problematic when the target is outside the envelope 100 (FIG. 1B). In the latter case, only a small number of transducers can reach the target area without reflection, and the closer the target is to the skull, the more transducers will be completely excluded from treatment due to reflection.
The treatment envelope 100 can be extended by reducing the frequency, e.g., to 0.2 MHz, and employing shear waves. Shear waves are largely absorbed in the skull at frequencies between 0.5 MHz and 4 MHz; at lower frequencies, however, their absorption is reduced to about that of longitudinal modes. Moreover, at 0.2 MHz, the sound velocity of shear waves in water (˜1500 m/s) is comparable to that in the skull (˜1400 m/s), thereby essentially eliminating the problem of reflection above a critical angle.
Previous methods of utilizing shear waves have calculated the phase shifts and amplitude attenuation associated with an originally longitudinal mode that is converted to a shear mode upon encountering the skull, and converted back to a longitudinal mode when entering the soft tissue of the brain. This approach is limited to large incidence angles at which no longitudinal mode is excited in the skull, or is otherwise inaccurate. In order to optimize focusing properties and maximize the amount of energy available in the focus, the coexistence of longitudinal and transverse modes ought to be taken into consideration.